Ferromagnetic metal (or alloy) powder has been used as a ferromagnetic material for magnetic recording media. The metal powder contains iron, nickel and/or cobalt as a main component. The ferromagnetic metal powder is dispersed in an organic binder such as vinyl chloride-vinyl acetate copolymer, styrene-butadiene copolymer, epoxy resin or polyurethane resin to prepare a ferromagnetic coating composition, and then the composition is coated on a non-magnetic support to provide a magnetic recording medium. The magnetic powder is unstable, and can be particularly easily oxidized in air, while it can provide a magnetic recording medium having higher density than that of oxide type magnetic powders. The metal is dangerous in powder form because it is combustible. This tendency is greater if the ferromagnetic metal powder contains an iron. If the metal powder is oxidized in air its saturation magnetization is lowered.
Various methods for preparing ferromagnetic metal powders having an improved oxidation stability have been proposed. For example, one method comprises immersing a ferromagnetic metal powder in an organic solvent, allowing it to stand for long time at room temperature to evaporate the solvent and to form a film of oxide and thermal-treating it in air (Japanese Patent Application (OPI) No. 54998/77). Another method comprises introducing an oxygen-containing gas in an organic solvent containing a ferromagnetic metal powder to oxidize it (Japanese Patent Application (OPI) No. 85054/77). Still another method comprises treating an active metal powder with an oxidizing agent such as permanganete (Japanese Patent Application (OPI) No. 112465/76). However, these methods are not sufficient because some of them require long periods of time to carry out their steps, and some of them use complicated steps. Further, sufficient stability to oxidation is not obtained.
It has been proposed in Japanese Patent Application (OPI) No. 155398/77 that a metal powder be wet with an organic solvent containing a silicone oil and dried at 200.degree. C. to 350.degree. C. This method involves simple steps and provides a strong protective film or layer on the surface of the metal powder, whereby an anti-oxidation property is provided to some degree. However, magnetic powders having sufficient stability to oxidation are still not obtained.